1. Field of the Invention
The present invention relates generally to an optical attenuator using an optical fiber employable in an optical fiber communication circuit system. More particularly, the present invention relates to an optical attenuator of the foregoing type which assures that reflected return light loss is minimized. Further, the present invention relates to a method and an apparatus for producing an optical attenuator of the foregoing type.
2. Description of the Related Art
As is well known by any expert in the art, various kinds of stationary type optical attenuators each usable for attenuating a quantity of light beam to pass through an optical fiber communication circuit have been hitherto put in practical use.
To facilitate understanding of the present invention, typical conventional optical attenuators will briefly be described below with reference to FIG. 5(a) and FIG. 5(b).
FIG. 5(a) is a sectional side view which illustrates by way of example the structure of an air gap type optical attenuator.
Ferrules 35 and 36 include central holes, respectively, through which optical fibers 33 and 34 with sheaths 31 and 32 removed therefrom are inserted. After completion of the inserting operation, the optical fibers 33 and 34 are immovably held in the corresponding central holes with the aid of an adhesive. Subsequently, end surfaces of both the ferrules 35 and 36 are ground and polished to assume a predetermined contour, respectively.
While the ferrules 35 and 36 are inserted into an alignment sleeve 37 together with the optical fibers 33 and 34 from the opposite sides of the alignment sleeve 37, a predetermined quantity of light attenuation is attained by properly adjusting a distance S between the end surfaces of the ferrules 35 and 36 to vary the quantity of light to be attenuated in proportion to the distance S.
Next, FIG. 5(b) is a sectional side view which illustrates by way of example the structure of a vaporized metal deposited film type optical attenuator. A stationary type optical attenuator of the aforementioned type has the optical attenuator shown in FIG. 5(a) utilized therefor. A glass plate 38 plated with a vaporized metal to assume a predetermined attenuation value is first placed in the interior of an alignment sleeve 37, and ferrules 35 and 36 are then inserted into the alignment sleeve 37 from the opposite sides of the same until the foremost ends of the ferrules 35 and 36 come in contact with the glass plate 38 (see Light Guide Digest, NO. 2, 1990, published by AT & T).
With respect to the conventional stationary type optical attenuators as mentioned above, a so-called reflected return light loss arises when an optical signal generated from a light source is reflected at the joint end surface of an optical fiber and then returns to the light source again. The loss has been hiterto noted as a common problem to conventional optical attenuators.
Since the reflected return light causes the light source to oscillate unstably, and moreover, it becomes a factor in inducing a noise signal, the reflected return light loss is a serious problem especially for an optical fiber communication circuit system having a large capacity for operating within a wide range.
It has been found that in a case where the end surface of an optical fiber is ground and polished at a right angle relative to an optical axis of the optical fiber, there arises a reflected return light loss of -10 dB (which corresponds to about 10%).
To solve the problem of referenced return light loss as mentioned above, a proposal has been made with respect to a method of slantwise grinding and polishing the end surface of an optical fiber with an angle of eight degrees or more as measured from a plane extending at a right angle relative to an optical axis of the optical fiber so as to allow reflected light to be introduced into a clad of the optical fiber while reducing the quantity of reflected light returned to the light source. When this method is employed, the reflected return light loss is reduced to a level of -60 dB (which corresponds to about 0.0001%). This means that the reflected return light loss can be reduced remarkably by employing the method as mentioned above.
However, it has been found that the conventional air gap type optical, attenuator has a problem that the quantity of multiple light reflections emitted from the surfaces of optical fibers located opposite to each other is liable to vary depending on the environmental temperature, and the aforementioned problem causes the light attenuation value of the optical attenuator to vary unstably.
In addition, it has been found that the conventional vaporized metal deposited film type optical attenuator has drawbacks in that it is difficult to control exactly production of each optical attenuator while maintaining a thickness of each film within a certain range to ensure a specified attenuation value. The film is easily injured or scratched when a ferrule is mounted on and dismounted from the optical attenuator, and moreover, there is not practically available any method of repairing an injured or scratched film no matter how it is scratched or injured.